Noise—substantially thermal noise and 1/f-noise—is superposed on every electrical payload signal. Additionally, noise—so-called residual noise (English: residual noise)—is generated in every device under test, which is additionally superposed on the already noisy electrical signal which is processed by the device under test.
In order to maximise the signal-noise interval of the electrical signal in a signal-processing unit, a measurement of the noise contained in the electrical payload signal is indispensable. In this context, a distinction must be made between an amplitude noise variable in amplitude and a phase noise variable in phase.
DE 10 2007 012 122 A1 discloses a method and a device for measuring the phase noise of an electrical signal which represents the prior art in phase-noise measurement technology. In this context, the phase noise contained in the measurement signal is separated from the measurement signal without noise by mixing the measurement signal in a mixer of a phase-locked loop (PLL) with a reference signal generated by the voltage-controlled oscillator of the phase-locked loop. In the controlled condition of the phase-locked loop, a reference signal, which corresponds to the measurement signal without phase noise, is obtained at the output of the voltage-controlled oscillator, while the phase noise of the measurement signal to be determined is disposed correspondingly at the output of a phase detector associated with the phase-locked loop.
In the phase-locked loop, only the phase noise contained within the measurement signal is unambiguously selectable and accordingly detectable, since the amplitude noise also contained in the measurement signal is suppressed or respectively attenuated by the mixer of the phase-locked loop and is therefore not available for an accurate measurement. Added to this, the phase-locked loop provides the disadvantageous property of suppressing phase noise in the environment of the carrier frequency of the measurement signal (PLL suppression), and accordingly, does not deliver an exact copy of the phase noise of the measurement signal over the entire measurement-frequency range. Finally, the phase-locked loop is typically provided with a relatively high time constant which disadvantageously determines a relatively low controlling or respectively settling of the phase-locked loop. In the case of relatively high phase jumps of the measurement signal—inter alfa, in the case of phase jumps >90°—a settling of the phase-locked loop is not at all possible.
A device for measuring the amplitude noise and phase noise which is generated in a device under test and superposed on the already present phase noise of the electrical signal processed by the device under test is known from US 2008/0075299 A1. In this context, the excitation signal is supplied in a first signal path to the device under test (DUT), while the same excitation signal is supplied in a second signal path to a series circuit comprising an adjustable attenuator and an adjustable phase shifter. The output signals of the first and second signal path are supplied to the two inputs of a 3-dB hybrid coupler, so that a carrierless signal is present at one output of the 3 dB hybrid coupler, and a signal amplified at its carrier frequency is present at the other output of the 3-dB hybrid coupler. The carrierless signal is supplied to an amplifier, and the signal amplified in its carrier frequency is supplied to a phase shifter, while the output signals of the amplifier and the phase shifter are mixed in a mixer. The signal at the output of the mixer represents the amplitude noise of the device under test if the phase shifter generates a phase of the signal amplified in its carrier frequency phase-synchronous to the phase of the amplified carrierless signal. The phase noise of the device under test is present at the output of the mixer if the phase shifter generates a phase of the signal amplified in its carrier frequency phase-shifted by 90° relative to the phase of the amplified carrierless signal.
The device of US 2008/0075299 A1 demands a very high-effort, processor-supported adjustment of the phase shifter and the attenuator. Additionally, a very high-quality signal source is required to generate the excitation signal for the first and second signal path with very good amplitude-noise and phase-noise properties. Finally, the amplitude noise and phase noise of the device under test cannot be measured simultaneously, since the second phase shifter must be reset in every case for the amplitude-noise measurement and phase-noise measurement.
An object of the invention is therefore to provide a method and a device for measuring simultaneously the amplitude noise and the phase noise of an electrical signal under test and/or a device under test which no longer provides the disadvantages named above.